Lamination is a general term which denotes the building up of thin sheets or layers of one or more materials (e.g. wood, paper, etc.) with the aid of a bonding material, usually a thermosetting resin, to form a thicker, heavier sheet or board having desirable properties.
Laminates are often prepared by a process which involves coating or impregnating a suitable covering material (e.g. paper or cloth) with a thermosetting resin (e.g. a phenolic resin). A sandwich is then made from one or more sheets of this impregnated covering material and a supporting substrate (e.g. hardboard). This sandwich is then heated under compressive forces (usually unidirectional) to form a hard, dense board or molded form (i.e. it is laminated).
Typically, the covering material is only bonded or laminated to one of the broad sides of the substrate, and the contiguous thin sides and remaining broad side are exposed (i.e. non-laminated). By contrast, when the thin sides (skewed, rounded or normal to the broad side to be laminated) are to be covered, such is ordinarly accomplished by first forming a shaped unsupported laminate in a mold and then gluing this molded form to the supporting substrate (i.e. indirect formation of a supported laminate). Alternatively, and less frequently, edge or side lamination can be obtained directly by complex operations involving the simultaneous application of surface and side pressures (e.g. vertical and lateral pressures).
Suitable covering materials include paper, asbestos paper, woven and non-woven fabrics (e.g. cotton and nylon fabrics), fiberglass, and the like. The more popular resins include phenol-formaldehyde resins, melamine-formaldehyde resins, ureaformaldehyde resins, epoxy resins, and polyester resins. Supporting substrates include wood, plywood, processed hardboard, and gypsum board.
Most commercially available laminating presses are of the unidirectional type. By unidirectional, it is meant that the compressive forces exerted by the press are exerted in only one direction or dimension (e.g. vertical). Ordinarily, one of two parallel, flat, paired press platens is held in a fixed position and compressive forces (i.e. pressure) are generated by moving the other press platen toward the first, thereby squeezing a laminating assembly or sandwich between the platens.
Different resins or plastic-forming systems require different conditions of heat and pressure for curing or hardening. Thus, some phenolic resins require rather severe conditions for curing or hardening (e.g. 1500 psig and 350.degree.F. for five minutes). Consequently, such resins cannot be used with supporting substrates having low compressive strengths (e.g. gypsum board). However, other resins are available which can be cured or hardened under much less severe conditions (e.g. certain unsaturated polyester resins can be cured at 280.degree.F. and 150 psig for five minutes).
Laminates of this general type are known and many of them are available commercially. Such laminates are used in the manufacture of furniture, counter-tops, doors, walls and the like. Consequently, the knowledge and selection of suitable covering materials (e.g. paper), suitable resins (e.g. unsaturated polyester resins) and suitable substrates (e.g. plywood), are within the skill of the art when aided by this disclosure.
Some of these prior art laminates have disadvantages which limit their usefulness.
First, some of the supporting substrates which can be used (e.g. 4 .times. 8 feet sheets of half-inch-thick plywood or gypsum board) exhibit minor but noticeable variations in width. Thus, walls made from these substrates (or made from "one-sided" laminates of these substrates) must be constructed in such a way as to compensate for these minor differences in width. In the case of laminated plywood panels, small strips or moldings (e.g. wood, metal or plastic) are frequently used to hide the seams formed with the thin sides of two laminates are placed in abutting relationship, as in the construction of a wall. Similarly, the seams formed in the construction of walls from ordinary gypsum board are hidden by the use of tape and plaster.
Laminated gypsum board is a recent development and it has been suggested that the non-laminated edges of the laminated gypsum board would be covered with a suitable molding, much in the same manner as plywood laminates. The preparation of laminated gypsum board is described in U.S. Pat. No. 3,692,620, which is incorporated herein by reference.
A second disadvantage of some conventional laminates is the susceptibility of such laminates to damage along their edges where the thin, non-laminated sides intersect the broad laminated surface, usually at right angles (i.e. normally).
A third disadvantage of some laminates is the unsightly absence of edge or side lamination. Consequently, such edges or sides must be disguised or covered in some way when such laminates are used to form, for example, a table or desk top.